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Why is Light Text Harder to Read Than Dark Text?Scharff and Ahumada (2002, 2003) measured text legibility for light text and dark text. For paragraph readability and letter identification, responses to light text were slower and less accurate for a given contrast. Was this polarity effect (1) an artifact of our apparatus, (2) a physiological difference in the separate pathways for positive and negative contrast or (3) the result of increased experience with dark text on light backgrounds? To rule out the apparatus-artifact hypothesis, all data were collected on one monitor. Its luminance was measured at all levels used, and the spatial effects of the monitor were reduced by pixel doubling and quadrupling (increasing the viewing distance to maintain constant angular size). Luminances of vertical and horizontal square-wave gratings were compared to assess display speed effects. They existed, even for 4-pixel-wide bars. Tests for polarity asymmetries in display speed were negative. Increased experience might develop full letter templates for dark text, while recognition of light letters is based on component features. Earlier, an observer ran all conditions at one polarity and then switched. If dark and light letters were intermixed, the observer might use component features on all trials and do worse on the dark letters, reducing the polarity effect. We varied polarity blocking (completely blocked, alternating smaller blocks, and intermixed blocks). Letter identification responses times showed polarity effects at all contrasts and display resolution levels. Observers were also more accurate with higher contrasts and more pixels per degree. Intermixed blocks increased the polarity effect by reducing performance on the light letters, but only if the randomized block occurred prior to the nonrandomized block. Perhaps observers tried to use poorly developed templates, or they did not work as hard on the more difficult items. The experience hypothesis and the physiological gain hypothesis remain viable explanations.
Document ID
20060020677
Acquisition Source
Ames Research Center
Document Type
Reprint (Version printed in journal)
Authors
Scharff, Lauren V.
(Austin State Univ. Nacogdoches, TX, United States)
Ahumada, Albert J.
(NASA Ames Research Center Moffett Field, CA, United States)
Date Acquired
August 23, 2013
Publication Date
September 23, 2005
Publication Information
Publication: Jountal of Vision
Volume: 5
Issue: 8
ISSN: 1534-7362
Subject Category
Documentation And Information Science
Funding Number(s)
CONTRACT_GRANT: NNA050544A
Distribution Limits
Public
Copyright
Other

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